Navigation Toolbar Bottom and other 85,000 icons from Icons8 icon pack follow the visual guidelines of the operating systems: Windows, iOS, Android KitKat, and Material. PNG format is free up to 100x100 px.
The icon is shaped like a rectangle with round edges. About a third of the way down there is a horizontal line running through it. In between the horizontal line and the base of the rectangle are two small circles followed by a long skinny rectangle.